1. Field of the Invention
The present invention is generally directed to a bone conduction device, and more particularly, to a mechanically amplified piezoelectric transducer.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive or sensorineural. In many people who are profoundly deaf, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. Various prosthetic hearing implants have been developed to provide individuals who suffer from sensorineural hearing loss with the ability to perceive sound. One such prosthetic hearing implant is referred to as a cochlear implant. Cochlear implants use an electrode array implanted in the cochlea of a recipient to bypass the mechanisms of the ear. More specifically, an electrical stimulus is provided via the electrode array directly to the cochlea nerve, thereby causing a hearing sensation.
Conductive hearing loss occurs when the normal mechanical pathways to provide sound to hair cells in the cochlea are impeded, for example, by damage to the ossicular chain to ear canal. However, individuals who suffer from conductive hearing loss may still have some form of residual hearing because the hair cells in the cochlea are may remain undamaged.
Individuals who suffer from conductive hearing loss are typically not candidates for a cochlear implant due to the irreversible nature of the cochlear implant. Specifically, insertion of the electrode array into a recipient's cochlea exposes the recipient to risk of the destruction of the majority of hair cells within the cochlea. The destruction of the cochlea hair cells results in the loss of all residual hearing by the recipient.
Rather, individuals suffering from conductive hearing loss typically receive an acoustic hearing aid, referred to as a hearing aid herein. Hearing aids rely on principles of air conduction to transmit acoustic signals through the outer and middle ears to the cochlea. In particular, a hearing aid typically uses an arrangement positioned in the recipient's ear canal to amplify a sound received by the outer ear of the recipient. This amplified sound reaches the cochlea and causes motion of the cochlea fluid and stimulation of the cochlea hair cells.
Unfortunately, not all individuals who suffer from conductive hearing loss are able to derive suitable benefit from hearing aids. For example, some individuals are prone to chronic inflammation or infection of the ear canal and cannot wear hearing aids. Other individuals have malformed or absent outer ear and/or ear canals as a result of a birth defect, or as a result of medical conditions such as Treacher Collins syndrome or Microtia. Furthermore, hearing aids are typically unsuitable for individuals who suffer from single-sided deafness (total hearing loss only in one ear). Cross aids have been developed for single sided deaf individuals. These devices receive the sound from the deaf side with one hearing aid and present this signal (either via a direct electrical connection or wirelessly) to a hearing aid which is worn on the opposite side. The disadvantage of this technology is the need for the individual to wear two hearing aids and suffer the complications of hearing aid use.
When an individual having fully functional hearing receives an input sound, the sound is transmitted to the cochlea via two primary mechanisms: air conduction and bone conduction. As noted above, hearing aids rely primarily on the principles of air conduction. In contrast, other devices, referred to as bone conduction devices, rely predominantly on vibration of the bones of the recipients skull to provide acoustic signals to the cochlea.
Those individuals who cannot derive suitable benefit from hearing aids may benefit from bone conduction devices. Bone conduction devices function by converting a received sound into a mechanical vibration representative of the received sound. This vibration is then transferred to the bone structure of the skull, causing vibration of the recipient's skull. This skull vibration results in motion of the fluid of the cochlea. Hair cells inside the cochlea are responsive to this motion of the cochlea fluid, thereby generate nerve impulses resulting in the perception of the received sound.
A known alternative to a normal air conduction aid is a bone conduction hearing aid which uses a hearing aid to drive a vibrator which is pushed against the skull via a mechanism, such as glasses or wire hoops. These devices are generally uncomfortable to wear and, for some recipients, are incapable of generating sufficient vibration to accurately present certain received sounds to a recipient.